1. Field of the Invention
This invention relates to radio communication systems such as mobile communication systems, for example, wireless LAN, a mobile telephone, etc.
2. Description of the Related Art
Recently, a wireless LAN network has been constructed and a hot-spot service providing the internet connection service has become popular. The “hot-spot” implies a radio communication area in which the service can be obtained. This service is often provided in, for example, cafes and restaurants to attract customers.
However, the radio communications area offering the service may extend out of the shops and stores, and persons outside them can obtain the service. For this reason, the communications may be intercepted or unauthorized access to the communications may be made To prevent the unauthorized access, encryption has been executed in the steps of WEP (Wired Equivalent Privacy), i.e. the wireless LAN standard based on IEEE 802.11. According to this method, however, there is a problem that the unauthorized access can be made for a long time by decrypting an encryption key.